Role of 4f electrons and 3d-4f hybridization in metal-insulator transition in RE (La, Nd, Sm, Eu, Dy and Er)-doped vanadium dioxide for thermochromic applications

Vanadium dioxide is a promising thermochromic material for novel applications to smart optoelectronic devices due to its reversible insulator-to-metal transition which is accompanied by rapid variation in its optical properties. In this work, rare-earth (La, Nd, Sm, Eu, Dy and Er) dopants substituting vanadium site were investigated in detail to control the phase transition temperature (TMIT) and optical properties of VO2. It is found that TMIT is mainly related to the nature of 3d-4f hybridization in RE-doped VO2(M1): the direct overlapping of 3d-4f orbitals contributes to V–V dimerization, but it is less pronounced to lower TMIT; the indirect 3d-4f interaction via O-2p makes 4f electrons itinerant to egπ states to assist MIT from M1 to R phase at lower temperatures. The dominating nature of 3d-4f hybridization varies with changing RE dopant's ionic radius and electronegativity. Er offered the lowest TMIT with a decreasing rate of 12.8 °C/at. % due to effective 4f-O2p-3d hybridization. RE dopants were also found effective in changing optical properties by scaling Eg1 and Eg2 bandgaps of VO2. Hence, the fashion of 3d-4f interaction in RE-doped VO2 is found to play important role in modulating thermochromic properties of VO2 for smart window applications and smart optoelectronics devices.